1. Field of the Invention
This invention relates to a method and an apparatus for treatment of wastewater, and more particularly, a method for biologically decomposing wastewater containing organic sulfur compounds such as dimetyl sulfoxide (hereinafter referred to DMSO), and an apparatus for carrying out such decomposition.
2. Description of the Prior Art
Accompanying recent introduction of DMSO as release agents and cleaning agents in the field of the electronic industry, needs for decomposing of DMSO are increasing. For such needs, the present inventors have developed a biological treatment method of decomposing DMSO, as described in Japanese Patent No. 2769973.
Generally employed as a method for biological treatment of wastewater containing organic compounds are fluidized-bed and fixed-bed biological treatment methods.
The fluidized-bed biological treatment method is carried out by using flowing carriers for the attachment of microorganisms flowing in a bioreactor. According to this type of the treatment, the excess microorganisms are discharged as suspended matters along with treated water. This poses a common problem that an equipment for removing suspended matters in the treated water is increased in size.
DMSO itself has a high permeability and acts as a release agent, so that where an organic compound contained in the wastewater is DMSO, the fluidized-bed biological treatment of such wastewater brings the flowing carriers into such a state as if they are washed by the releasing agent under the continuous flow of the flowing carriers in the bioreactor. Consequently, there arises a problem inherent to the treatment of the wastewater containing DMSO that microorganisms are relatively easily released from the carriers as compared with the treatment of wastewater containing a different organic matter.
On the other hand, the fixed-bed biological treatment of wastewater containing organic sulfur compounds such as DMSO causes generation of sulfuric acid and hence lowers the pH due to an organic sulfur compound contained in the wastewater, as the decomposition of the organic sulfur compounds proceeds.
The fixed-bed biological treatment also provides a plug flow of wastewater, so that the pH is lowered as the wastewater fed into a wastewater inflow region moves through the fixed bed to be subjected to the biological treatment.
Because of this, as the biological treatment of wastewater containing organic sulfur compounds proceeds, the pH is lowered and unevenly distributed within the apparatus even if the pH of the wastewater at the wastewater inflow region is adjusted to a pH suitable for the biological treatment. Consequently, the apparatus may have a deteriorated biological treatment condition and hence is unlikely to obtain a high treatment capability.
As a result of insufficient treatment capability, there arises a problem that the organic sulfur compounds may not be sufficiently decomposed, resulting in the generation of odor.
The aforesaid Japanese Patent No. 2769973 addresses the lowering of the pH by partly circulating treated water and stabilizing the pH. To do this, however, an equipment for circulation of treated water must be installed.
In either case, the problems involved in both the fluidized-bed and fixed-bed biological treatments may not be solved even by the invention of the Japanese Patent No. 2769973.
It is an object of the present invention to provide a method and apparatus for treatment of wastewater containing organic sulfur compounds that achieves an excellent treatment efficiency, which may not be available through a conventional method or apparatus, while omitting the necessity to install an equipment for circulating a part of treated water, and solving problems involved in employing the conventional fluidized-bed or fixed-bed biological treatment method.